The present invention relates to a surface acoustic wave filter capable of exhibiting low insertion loss and ripple-reduced frequency characteristics notwithstanding of no use of inductor as a phase shifter and regardless of electrode array which can not necessarily assure perfect unidirectionality.
A unidirectional electrode array for the surface acoustic wave filter is reported, for example, in Yamanouchi et al's article entitled "Low Insertion Loss Elastic Surface Wave Filter Using Group-Type Unidirectional Interdigital Transducer" contained in Ultrasonic Wave Study Group Data "US75-15" published by The Institute of Electronics and Communication Engineers of Japan on 1975. In this known surface acoustic wave filter, an electrical phase difference of .pi./2 rad. is produced by a phase shifter, while the electrode array is so designed that the geometrical phase difference of 3.pi./2 rad. can be established. With this structure of the filter, the surface acoustic wave can travel only in one direction, whereby improved filter characteristics are realized. The phrase "geometrical phase difference" means the phase difference which is determined by the inter-center distance between the electrodes. A meander electrode is disposed between the sending electrode and the reflecting electrode in a meandering pattern. When the sending electrode and the reflecting electrode are each implemented in a so-called split electrode configuration, there can be formed the geometrical phase difference of 3.pi./2 rad. by such an electrode arrangement in which one finger of the meander electrode having a finger width of .lambda..sub.0 /8 (where .lambda..sub.0 represents the wavelength of the surface acoustic wave at the center frequency of the filter) is disposed between the sending electrode and the reflecting electrode while three fingers of the meander electrode each having the width of .lambda..sub.0 /8 are disposed between other reflecting and sending electrodes. The split electrode is effective for suppressing the undesired waves produced due to discontinuity in the acoustic characteristic impedance at the boundary between a region formed with the finger and a region without finger.
Another type of the elastic surface wave filter is disclosed in JP-A-60-117907.
In either of the surface acoustic wave filters known heretofore, an inductive element is used as the phase shifter for producing the phase difference. For this reason, the prior art elastic surface wave filters are disadvantageous in manufacturing the filter in a miniature size on the mass production basis and besides suffers from a problem that noise is likely to be produced through electromagnetic induction.